Optical isolators (sometimes called “optical diodes”), and particularly polarization-dependent optical isolators (also referred to as “Faraday isolators”) are commonly utilized optical components which allow the transmission of light in only one predetermined direction, in essence serving is a one-way light valve, permitting radiation to pass through one way and not the other. Optical isolators are typically used to prevent back reflections/unwanted feedback into an optical oscillator, such as a laser cavity. The operation of the device depends on the Faraday effect achieved by the optical isolator's main component—the Faraday rotator. In summary, a magnetic field, B, applied to a Faraday rotator causes a rotation in the polarization of the light passing through the rotator at a predefined angle β, due to the Faraday effect. The value of this angle of rotation β, is determined by the following expression:β=Bd,
where, v is the Verdet constant of the material (amorphous or crystalline; solid, liquid, or gaseous) of which the rotator is made, and d is the length of the rotator.
Referring now to Prior Art FIG. 3, a conventional optical “Faraday” isolator 300 is shown by way of example. The prior art isolator 300, is typically positioned between two optical elements 302a at its first end A, and 302b at its second end B. One or both of the elements 302a, 302b may be optical fibers, and/or other optical devices/components. The prior art isolator 300 includes an input an input linear polarizer 304a at first end A, a magnetic rotator 306 based on the Faraday effect (e.g., a Faraday rotator), and an output linear polarizer 304b whose pass axis is rotated 45° relative to the input polarizer 304a. A magnetic field 308 surrounding the rotator 306 is chosen for the wavelength of the desirable light so that the plane of polarization within the rotator 306 is rotated by 45° into the output polarizer 304b pass direction (toward second end B).
Light 310 travelling in the forward direction between ends A and B becomes polarized linearly by the input polarizer 304a so that only light 314a of a desired polarization orientation exits the input polarizer 304a and enters the Faraday rotator 306 (with the other undesired polarization component 312 of the input light 310 being dissipated, deflected, or otherwise prevented from exiting the input polarizer 304a into the rotator 306). The Faraday rotator 306, under the influence of the predetermined magnetic field 308, then rotates the polarization of the light 314a by 45 degrees to produce rotated polarized light 314b at its output, which then enters the output polarizer 304b and then freely passes therethrough to the optical element 302b. 
Light 316 travelling in the backward direction (from end B to end A) becomes linearly polarized at 45 degrees by the output polarizer 304b to produce the polarized light 320 (with the other undesired polarization component 318 of the backward traveling light 316 being dissipated, deflected, or otherwise prevented from exiting the output polarizer 304b into the rotator 306). The Faraday rotator will again rotate the polarization of the polarized light 320 by 45 degrees to produce the rotated polarized light 322 at the interface between the rotator 306 and the input polarizer 304a. This means that the light 322 is polarized orthogonally with respect to the polarization orientation of the input polarizer 304a, and thus the light 322 is dissipated, deflected, or otherwise prevented from exiting the input polarizer 304a toward the optical element 302a. 
While in essence, the above-described commonly known prior art optical isolator generally serves its purpose well in most commercial/industrial/scientific applications, such isolators suffer from a number of significant disadvantages. First, the input and output polarizers, and especially the Faraday rotator are “bulk” optical components that are quite large as compared to sizes of optical fibers to which they are most often connected. As separate bulk optical components, they must be properly aligned and tuned prior to and during installation.
Conventional optical isolators are also relatively expensive to manufacture, use, maintain and/or to troubleshoot. Furthermore, there are a total of six points of contact interfaces between the input optical element, the input polarizer, the rotator, the output polarizer, and the output optical element—each of these interface points are a potential source of signal loss and/or damage for the entry/exit interfaces of each prior art isolator elements. While attempts to address this issue, by coating each glass interface surface with anti-reflective coating or by tilting the optical components to form sufficient angles between interface surfaces to reduce/avoid back reflection, from an optical power-handling viewpoint, the interfaces where light signals exit and enter glass surfaces remain the most likely candidates for damage from the signals. Furthermore, if used in systems with unpolarized light signals conventional isolators must be connected to yet additional separate bulk polarization components on each end. Finally, the multi-element construction and need to maintain proper alignment between all the elements makes conventional optical isolators unstable and unreliable in conditions where such isolators may be subjected to physical stress and/or to temperature variations.
It would thus be desirable to provide an optical isolator device of a substantially in-fiber physical profile to facilitate installation and utilization thereof. It would also be desirable to provide an in-fiber optical isolator device that is preconfigured with desired characteristics and that does not require tuning. It would moreover be desirable to provide an in-fiber optical isolator device of high physical and thermal stability, and resistance to effects of physical stress and/or temperature variations on the performance thereof. It would also be desirable to provide an in-fiber optical isolator device without undesirable and damage-prone inter-element interface points. It would additionally be desirable to provide an in-fiber optical isolator device that is easy and inexpensive to manufacture, install, use, and maintain.